1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory.
2. Description of the Related Art
An NAND-type flash electrically erasable programmable read only memory (EEPROM) is known as a nonvolatile semiconductor memory which is electrically rewritable, and which can be highly integrated (for example, see Japanese Patent Application Publication 2008-187051).
A nonvolatile semiconductor memory disclosed in the Patent Application Publication 2008-187051 has a memory cell array in which multiple NAND strings are arranged in an array. Each of the NAND strings is configured as follows. In each NAND string, multiple memory cell transistors are connected together in series. Each of the memory cell transistors is formed of an N-channel MOS transistor including a floating gate electrode and a control gate electrode stacked one on the other. The drain of an endmost one of the multiple cell transistors is connected to a bit line by a bit line contact via a selection N-channel metal oxide semiconductor (MOS) transistor. The source of the other-endmost one of the multiple cell transistors is connected to a source line by a source line contact via another selection N-channel MOS transistor.
When data is written to the nonvolatile semiconductor memory, a high voltage is applied to control gate electrodes respectively of selected memory cell transistors, whereas an intermediate voltage is applied to NAND strings which include non-selected memory cell transistors. For this reason, an inversion voltage between the NAND strings needs to be set higher. To this end, as an element isolating layer, a shallow trench isolation (STI) is formed between each two adjacent NAND strings. The STI is made by filling an insulating material into its trench.
The element isolating capability of the STIs in the nonvolatile semiconductor memory disclosed in the Patent Application Publication 2008-187051 depends on the distance between the two adjacent elements (memory cells), namely the width of the element isolation insulating film (the width of the trench), and the depth of the element isolation insulating film (the depth of the trench).
When the width of the element isolation insulating film is narrowed for miniaturizing the nonvolatile semiconductor memory, the depth of the element isolation insulating film needs to be made deeper for securing a sufficient element isolating capability. This means that the aspect ratio of the trench needs to be increased. The increase in the aspect ratio makes it very difficult to fulfill an etching process, a process for filling the insulating material into the trench, and the like when the trench is formed.
This makes it difficult to narrow the pitch of the NAND strings, and thus causes a problem of difficulty in high integration and high storage capacity.